Why should I make a copy of GTA5, I bought it on steam and started playing it with mods, but every video giving a tutorial of how to download mods always say to make a copy of GTA5, why? what does it do and how do you switch between them?

Hi, I recently downloaded the Back to the future mod for GTA5 and I did everything correctly, however whenever I start up gta5 is just crashes in the loading screen how do I fix it?

Hi, I just downloaded the simple zombies mod for GTA5 and it works fine, I started the infection however whenever I get close to any living person, zombie, survivor they just instantly die on the spot, I don't know how to fix it, maybe another plugin is doing this idk, can someone help me, thanks.